In nuclear reactors, particularly of the pressurized water type, use is made of control rods suspended from shafts guided vertically in guide tubes which are fixed at their lower end to adaptor tubes passing through the reactor cover, and are closed in their upper part so as to preserve the integrity of the pressurized enclosure. Each tube is associated with mechanisms permitting the shaft to move vertically to raise or lower the corresponding control rod.
The position of each control rod, and consequently of the suspension shaft, must be located continuously, and various devices are employed for this purpose. These devices must of course operate without contact through the wall of the adaptor tube and the use of ultrasonic signals for this purpose has already been proposed.